1. Field of the Invention
The present invention relates to an antilock brake control system for motorcycles, and more particularly, to an antilock brake control system for use in motorcycles on which the front and rear brakes can be separately operated.
2. Description of the Prior Art
Generally, the antilock brake control system equipped in the motorcycle operates such that: the velocity of each wheel and the estimated vehicle velocity are calculated using the wheel velocity sensors installed on the front and rear wheels; the wheel velocity is compared with the estimated vehicle velocity; and a signal causing the brake pressure of the wheel to drop is produced when the wheel velocity becomes less than a specified value, i.e., when the wheel slippage exceeds a predetermined level.
When the brakes are suddenly applied during vehicle travel, excessive slipping of the wheel to which the brake was applied is caused, resulting in a loss of steering stability and a dangerous situation. Therefore, antilock brake control systems which as conventionally proposed detect the generation of excessive slipping or the symptoms thereof, and control and suppress the brake pressure regardless of the degree of brake pedal or brake lever depression.
Ideally, when the difference between the true vehicle velocity and the velocity of each wheel is obtained, this difference expresses the true amount of slipping, and antilock brake control is applied based on this slipping amount. However, detection of the true vehicle velocity requires sophisticated precision machinery, which is not used in conventional antilock brake control systems due to the size and cost thereof, and the estimated vehicle velocity as estimated from the velocity of each wheel is therefore used.
In other words, the antilock brake control systems of this type operates such that: the wheel velocity of each wheel is calculated from the detection signals output from the wheel velocity sensor mounted at each wheel; the estimated vehicle velocity is calculated from each wheel velocity; the estimated vehicle velocity is compared with each wheel velocity; and when a wheel velocity becomes less than the estimated vehicle velocity by a predefined amount, i.e., the slipping index becomes greater than a predetermined value, it is determined that the wheels have a tendency to lock and an antilock signal is generated to lower the brake pressure of that wheel.
When the above-mentioned antilock brake control system is installed on a motorcycle, because the brakes on the front and rear wheels can be independently operated, the deviation of the estimated vehicle velocity from the actual vehicle velocity fluctuates greatly between those cases in which the brakes are applied to both wheels and when the brakes are applied to only one wheel.
For examples, as shown in FIG. 1, when the brakes are not applied to the front wheel, the front wheel has virtually no slipping with respect to the road surface, and accordingly, the estimated vehicle velocity (the single dot-dash line) obtained from the velocity of the front wheel is almost identical to the actual vehicle velocity (solid line). On the other hand, when the front brakes are applied, the front wheel slips on the road surface, and the estimated vehicle velocity (the dotted line) obtained from the velocity of the front wheel is significantly less than the actual vehicle velocity.
Focusing on this tendency, an improved antilock brake control circuit is proposed and is disclosed, for example, in U.S. Pat. No. 4,420,191 (corresponding to Japanese Patent Publication 61-28539). According U.S. Pat. No. 4,420,191, the control circuit switches, for a given period of time after the wheel deceleration of one wheel becomes greater than a predetermined value and it is detected that the brakes are sufficiently applied, to a a greater value with which the velocity of this wheel is compared for carrying out the slip detection of the other wheel.
Thus, according to the control circuit of U.S. Pat. No. 4,420,191, the antilock brake control for the rear wheel is carried out in the same manner, for a given amount of rear wheel slippage, regardlessly of the brakes applied to the front wheel.
However, the following problems arise in the above described antilock brake control system for motorcycles, because the detection of the front brake condition only occurs after the brakes are sufficiently applied.
When the front brakes are applied but are applied slowly, it may happen that the front wheel brake condition remains undetected and front wheel control is not applied. In a case like this, the estimated vehicle velocity obtained from the wheel velocity of the front wheel deviates from the actual vehicle velocity and is shown to be less than the actual (single dot-dash line in FIG. 2). However, because it is assumed that the brakes are not applied to the front wheel, the threshold set for rear wheel control is set at a value (threshold TH2) with a large deviation .DELTA.2 from the estimated vehicle velocity, a level less than the level (threshold TH1) which should normally be set. Therefore, the antilock brake control is applied to the rear wheel after locking of the rear wheel has significantly advanced, thus resulting in greater than necessary slipping and the inability to maintain stable control.
Note that the preceding description considers control of the rear wheel based on the front wheel, but the same problem arises when front wheel control is based on the rear wheel.